rfc: optionally hardlink packs for local branching

[Martin Pool]

I'd like to add an option to branch (and maybe push/pull) that will
hardlink packs between repositories when possible.

Normally regular push/pull/branch filter and copy only the relevant
reachable data, and this obviously would not.  Still, I think it would
be a useful option for some cases, and possibly much faster when it is
relevant.

-- 
Martin